Chemical vapor deposition (CVD) is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor phase reactants that contain desired components. The gases are introduced into a reactor vessel, and decompose and react at a heated surface on the wafer to form the desired film. Chemical vapor deposition is but one process of providing thin films on semiconductor wafers, such as films of elemental metals or compounds. It is a favored deposition process in many respects, principally because of its ability to provide highly conformal layers even within deep contacts and other openings.
Organic compounds are commonly utilized as chemical vapor deposition precursors. One subclass of this group which is finding increasing use in chemical vapor deposition of metals and metal compounds is organometallic/metal-organic precursors. Specifically, an example is the reaction of a titanium metal-organic precursor of the formula Ti(N(CH.sub.3).sub.2).sub.4, named tetrakisdimethyl-amidotitanium (TDMAT), and ammonia in the presence of a carrier gas which reacts to produce TiN according to the following formulla: EQU Ti(NR.sub.2).sub.4 +NH.sub.3 .fwdarw.TiN+organic by-products
Organometallic and metal-organic compounds contain a central or linking atom or ion (Ti in TDMAT) combined by coordinate bonds with a definite number of surrounding ligands, groups or molecules, at least one of which is organic (the (N(CH.sub.3).sub.2 groups in TDMAT). The central or linking atom as accepted within the art may not be a "metal" in the literal sense. As accepted within the art of organometallic compounds, the linking atom could be anything other than halogens, the noble gases, H, C, N, O, P, S, Se, and Te. The terms "organometallic" and "metal-organic" are often used by those in the art interchangeably, although there is a technical distinction between the two. Technically in an "organometallic", the "metal" is bonded to at least one carbon atom of the ligand. In a "metal-organic", the "metal" is bonded directly to atoms other than carbon in the ligand.
The above and other chemical vapor deposition reactions involving organometallics/metal-organics are typically conducted at low pressures of less than 1 Torr. It is typically desirable in low pressure chemical vapor deposition processes to operate at as low a pressure as possible to assure complete evacuation of potentially undesirable reactive and contaminating components from the chamber. Even small amounts of these materials can result in a significant undesired increase in film resistivity. For example, oxygen incorporation into the film before and after deposition results in higher resistivity. Additionally, it is believed that organic incorporation (specifically pure carbon or hydrocarbon incorporation) into the resultant film reduces density and increases resistivity Such organic incorporation can result from carbon radicals from the organic portion of the precursor becoming incorporated into the film, as opposed to being expelled with the carrier gas. Carbon incorporation can also cause other undesired attributes in the deposited film, such as low density which causes oxygen incorporation into the film when exposed to ambient air.
Hydrogen is a known capable reactant with deposited carbon or metal carbides. Such will react with carbon atoms to form volatile hydrocarbons. Hydrogen atoms, radicals or ions are more reactive than molecular hydrogen in producing volatile hydrocarbons.
It would be desirable to improve upon these and other prior art chemical vapor deposition processes in producing layers having minimal incorporated carbon.